NanoTin Catalysts for Electrochemical Reduction of Carbon Dioxide to Formate

ABSTRACT

High surface area tin oxide nanoparticles prepared by a facile hydrothermal method followed by electroreduction to tin act as electrocatalysts toward CO 2  reduction to formate, in some embodiments. At certain of these nano-structured tin catalysts, CO 2  reduction occurs selectively to formate at low overpotentials and with high Faradaic efficiencies, with high stability and significant current densities.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit under 35 U.S.C. §119(e) of U.S. Provisional Patent Application Ser. No. 61/983,581, filed Apr. 24, 2014, having the title, “NanoTin Catalysts for Electrochemical Reduction of Carbon Dioxide to Formate,” the disclosure of which is incorporated herein by reference in its entirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was made with government support under Grant No. DE-SC0001011 awarded by the Department of Energy. The U.S. Government has certain rights in the invention.

FIELD OF INVENTION

This invention relates to nanoparticles comprising tin that function as catalysts, such as for example, in the reduction of carbon dioxide to formate.

BACKGROUND OF THE INVENTION

The increasing carbon dioxide (CO₂) content in the atmosphere has been cited as a major contributor to the greenhouse effect and global warming. Electrochemical or photoelectrochemical reduction of CO₂ could provide an attractive solution to this climate issue with CO₂ converted into useful fuels and utilized as a chemical feedstock. It is also an energy storage strategy with solar or electrical energy stored as chemical energy in a reduced carbon product. Formic acid, produced from CO₂ reduction to the formate, is a good candidate as an anode fuel for fuel cells and a promising material as a hydrogen carrier. A number of homogeneous and heterogeneous catalysts have been evaluated for electrochemical CO₂ reduction including metal electrodes. Electrodes of Pb, Hg, In, Cd, and TI are able to promote reduction of CO₂ to formate but are highly toxic, expensive, or both.

SUMMARY OF THE INVENTION

We report here the results of an electrochemical study on the reduction of CO₂ at ultrafine nano tin oxide with controlled particle sizes reduced to contain metallic tin. These nano tin oxide particles were synthesized by utilizing a hydrothermal synthesis method. To maximize surface area, they were loaded onto high surface area carbon supports (carbon black and graphene) with the added advantage of utilizing their 3D porous structures to facilitate CO₂ transport and reduction. The as-prepared nano tin oxide surfaces were characterized as tin(IV) oxide (SnO₂) and shown to be quickly and efficiently reduced to the metal at an onset potential of about −1 V vs. SCE in aqueous 0.1 M NaHCO₃. We find a notable size dependence on CO₂ reduction efficiency to formate at the reduced nano tin oxide (i.e., metallic tin) catalyst surfaces. In one embodiment, efficiencies were maximized on 5 nm particles reaching a maximum Faradaic efficiency for formate production of >93%. These catalysts can be very stable during electrolysis and can continue producing formate for at least 18 hours. Electrocatalytic activity toward CO₂ reduction can be tuned by morphology and electronic structure of the Sn catalyst.

From our study, we have invented a technology that comprises several embodiments.

In one embodiment of the present invention, nanoparticles of tin oxide, associated with high surface area carbon appear. Other embodiments provide nanoparticles comprising tin, associated with high surface area carbon. Still other embodiments relate to an electrically-conductive surface in electrical communication with nanoparticles comprising tin, associated with high surface area carbon. Further embodiments involve that electrically-conductive surface in the context of an electrocatalytic electrode. Still further embodiments place that electrocatalytic electrode in an electrochemical cell.

Additional embodiments relate to methods of making nanoparticles of tin oxide, associated with high surface area carbon, comprising reacting a tin halide precursor such as SnCl₂, for example, in the presence of high surface area carbon, to form the nanoparticles of tin oxide, associated with high surface area carbon. Certain embodiments involve methods of making nanoparticles comprising tin, associated with high surface area carbon, comprising reducing nanoparticles of tin oxide, associated with high surface area carbon, thereby making the nanoparticles comprising tin, associated with high surface area carbon. Still other embodiments of the present invention relate to methods for making an electrocatalytic electrode, comprising: depositing, on an electrically-conductive surface, nanoparticles of tin oxide associated with high surface area carbon; reducing at least some of the tin oxide to form nanoparticles comprising tin, associated with high surface area carbon, thereby making the electrocatalytic electrode.

Yet additional embodiments relate to methods for reducing carbon dioxide to formate, comprising: providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises:

nanoparticles comprising tin, associated with high surface area carbon;

exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition; applying a catalyzing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react; thereby reducing the carbon dioxide to formate.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1a and 1b depict TEM images of one embodiment of the invention, an example of nano tin oxide on carbon black.

FIG. 2 portrays a high resolution Sn3d XPS spectrum of an example of nano tin oxide on carbon black.

FIG. 3 plots a Raman spectrum of an example of nano tin oxide on carbon black.

FIG. 4 displays single reductive linear sweep voltammetric (LSV) scans at 50 mV s⁻¹ under N₂ (dashed line) and in a CO₂ (1 atm, solid line) saturated solution in aqueous 0.1 M NaHCO₃ at bare glassy carbon electrode.

FIG. 5 displays single reductive linear sweep voltammetric (LSV) scans at 50 mV s⁻¹ under N₂ (dashed line) and in a CO₂ (1 atm, solid line) saturated solution in aqueous 0.1 M NaHCO₃ at glassy carbon electrode with carbon black.

FIG. 6 displays single reductive linear sweep voltammetric (LSV) scans at 50 mV s⁻¹ under N₂ (dashed line) and in a CO₂ (1 atm, solid line) saturated solution in aqueous 0.1 M NaHCO₃ at electrodeposited tin particles.

FIG. 7 displays single reductive linear sweep voltammetric (LSV) scans at 50 mV s⁻¹ under N₂ (dashed line) and in a CO₂ (1 atm, solid line) saturated solution in aqueous 0.1 M NaHCO₃ at reduced nano-SnO₂/carbon black.

FIG. 8 displays applied electrolysis potential dependence of total current densities (squares) and Faradaic efficiencies (circles) for formate production on reduced nano-SnO₂ loaded on carbon black.

FIG. 9 plots particle size dependence of Faradaic efficiencies for CO₂ reduction to formate on Sn catalysts.

FIG. 10 shows single oxidative LSV scans at 50 mV s⁻¹ in N₂ saturated 0.1 M NaOH at Sn catalysts of varying morphologies illustrating surface adsorption of OH⁻ accompanying oxidation to surface tin oxide.

FIG. 11a displays a TEM image, and FIG. 11b displays a high resolution TEM image showing obvious lattice fringe.

FIG. 12 plots Sn3d XPS spectrum of SnO₂ nanoparticles on graphene.

FIG. 13 plots single reductive LSV scans (d) at 50 mV s⁻¹ under N₂ (thin line) and in a CO₂ (thick line) saturated solution at reduced nano-SnO₂/graphene in aqueous 0.1 M NaHCO₃.

FIGS. 14a and 14b display high resolution TEM images for tin oxide nanoparticles on carbon black.

FIG. 15 presents a schematic illustration of a proposed mechanism responsible for the formation of SnO₂ nanoparticles by hydrothermal method.

FIG. 16 presents several physical characterizations of an embodiment, namely a of nano-SnO₂/carbon black sample: (a) survey X-ray photoelectron spectroscopy (XPS) spectrum; high resolution O1s XPS spectrum (b), C 1s spectrum (c), and valence band (d).

FIG. 17 presents an XRD pattern for an example of nano-SnO₂ on carbon black.

FIG. 18 presents an enlarged scattering Raman spectrum of an example of nano-SnO₂ on carbon black.

FIG. 19 plots initial and steady single reductive linear sweep voltammetric (LSV) scans of nano SnO₂/carbon black in N₂ saturated 0.1 M NaHCO₃ at the scanning rate of 50 mV s⁻¹.

FIG. 20 presents single reductive linear sweep voltammetric (LSV) scans with scan rate of 50 mV s⁻¹ at tin foil in N₂ (thin line) and CO₂ (thick line) saturated 0.1 M NaHCO₃ solution.

FIG. 21 presents controlled potential electrolysis at −1.8 versus SCE at nano-SnO₂/carbon black sample in 0.1 M NaHCO₃ solution with CO₂ flow.

FIG. 22 TEM images of nano SnO₂/carbon black after controlled potential electrolysis.

FIG. 23 plots single reductive LSV scans at nano SnO₂/carbon black sample before and after bulk electrolysis.

FIGS. 24 and 25 display SEM images of Sn foil.

FIG. 26 displays an SEM image of 200 nm Sn nanoparticles prepared by electrochemical reduction.

FIG. 27 displays a TEM image of 10 nm SnO₂ nanoparticles on carbon black.

FIG. 28 displays a TEM image of 5 nm SnO₂ nanoparticles on carbon black.

FIG. 29 displays a TEM image of 3 nm SnO₂ nanoparticles on carbon black.

FIG. 30 plots single reductive LSV scans (a) at 5 nm reduced tin oxide catalyst in different concentrated CO₂ saturated NaHCO₃ electrolyte, where total concentration of Na⁺ was kept at 0.2 M by adding NaClO₄.

FIG. 31 plots NaHCO₃ concentration dependence (b) of current density at −1.8 V.

FIG. 32 depicts a Tafel plot for production of formate at 5 nm SnO₂ sample based on the data in FIG. 8, for production of formate at 5 nm SnO₂.

FIGS. 33a and 33b provide TEM images of a nano-SnO₂/graphene sample.

FIG. 34 provides XRD (a) and survey XPS spectra (b) of a nano-SnO₂/graphene sample.

FIG. 35 depicts controlled potential electrolysis at −1.8 vs. SCE at the reduced nano-SnO₂/graphene in 0.1 M NaHCO₃ solution with CO₂ flow.

DETAILED DESCRIPTION

As required, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. The figures are not necessarily to scale, and some features may be exaggerated to show details of particular components. Therefore, specific structural and functional details disclosed herein are not to be interpreted as limiting, but merely as a basis for the claims and as a representative basis for teaching one skilled in the art to variously employ the present invention.

Unless defined otherwise, all technical and scientific terms used herein have the same meaning as is commonly understood by one of ordinary skill in the art to which this disclosure belongs. In the event that there is a plurality of definitions for a term herein, those in this section prevail unless stated otherwise.

Where ever the phrase “for example,” “such as,” “including” and the like are used herein, the phrase “and without limitation” is understood to follow unless explicitly stated otherwise. Similarly “an example,” “exemplary” and the like are understood to be non-limiting.

The term “substantially” allows for deviations from the descriptor that don't negatively impact the intended purpose. Descriptive terms are understood to be modified by the term “substantially” even if the word “substantially” is not explicitly recited.

The term “about” when used in connection with a numerical value refers to the actual given value, and to the approximation to such given value that would reasonably be inferred by one of ordinary skill in the art, including approximations due to the experimental and or measurement conditions for such given value.

The terms “comprising” and “including” and “having” and “involving” (and similarly “comprises”, “includes,” “has,” and “involves”) and the like are used interchangeably and have the same meaning. Specifically, each of the terms is defined consistent with the common United States patent law definition of “comprising” and is therefore interpreted to be an open term meaning “at least the following,” and is also interpreted not to exclude additional features, limitations, aspects, etc. Thus, for example, “a device having components a, b, and c” means that the device includes at least components a, b and c. Similarly, the phrase: “a method involving steps a, b, and c” means that the method includes at least steps a, b, and c.

“Nanoparticles of tin oxide” relate to particles, dispersed or agglomerated, comprising tin oxide. In some cases, a majority of the mass of the particle is tin oxide. In other cases, the particle is substantially tin oxide. The nanoparticles can be any suitable size. For example, the particles can have an average size of about 1 μm or less, about 200 nm or less, about 10 nm or less, or range in size from about 3 nm to about 10 nm. In some cases, the nanoparticles have an average size of about 5 nm. Certain instances provide amorphous, semi-crystalline, or nanocrystalline tin oxide. Certain other instances provide rutile tin oxide.

“Nanoparticles comprising tin” relate to particles, dispersed or agglomerated, comprising tin. In some cases, a majority of the mass of the particle is tin. In other cases, the particle is substantially tin. In still other cases, the particle contains tin and tin oxide. The nanoparticles can be any suitable size. For example, the particles can have an average size of about 1 μm or less, about 200 nm or less, about 10 nm or less, or range in size from about 3 nm to about 10 nm. In some cases, the nanoparticles have an average size of about 5 nm.

When nanoparticles of tin oxide or nanoparticles comprising tin are “associated with high surface area carbon,” the nanoparticles are in contact with the carbon. The nature of that contact may involve any suitable interaction. Covalent bonding, ionic bonding, van der Waals interactions, electrical communication, or any combination thereof is possible. In some cases, the association happens because the particle is synthesized in the presence of the high surface area carbon.

“Fluid compositions” include any suitable form of matter that can flow. Gasses, liquids, sols, slurries, gels with a mobile fluid phase, and combinations thereof are possible.

Some embodiments relate to an electrocatalytic electrode, comprising an electrically-conductive surface in electrical communication with nanoparticles comprising tin, associated with high surface area carbon. In some cases, the high surface area carbon comprises carbon black, graphene, or a combination thereof. In other cases, the high surface area carbon is chosen from carbon nanotubes, carbon black, mesoporous carbon, graphite, graphene, and combinations of two or more thereof. Certain instances provide that the electrically-conductive surface comprises glassy carbon, carbon paper, carbon cloth, or a combination thereof. Further instances involve the electrically-conductive surface comprising a gas diffusion electrode. Additional instances place the electrocatalytic electrode in an electrochemical cell adapted for the reduction of CO₂, such as, for example, a flow cell.

Further embodiments relate to making nanoparticles of tin oxide associated with high surface area carbon, comprising reacting tin(II) chloride in the presence of high surface area carbon and water for a time. In some instances, the reacting takes place at an increased temperature. Any suitable temperature can be used. In some cases, the temperature is greater than about 50° C., greater than about 100° C., greater than about 150° C., greater than about 200° C., or greater than about 250° C. In certain cases, the temperature is about 196° C. The time for reacting is any suitable time. That time can be at least 30 minutes, at least 3 hours, or no more than about 6 hours.

Reducing tin oxide to form tin can occur according to any suitable method. Contact with a reducing agent, application of a reducing potential, or combinations thereof are possible. In some cases, the reducing comprises applying a reducing potential to the tin oxide no more positive than about −1 V versus SCE, or no more positive than about −1.1 V versus SCE.

Certain embodiments of the present invention involve methods for making an electrocatalytic electrode. In some cases, nanoparticles of tin oxide associated with high surface area carbon are deposited according to any suitable method onto an electrically-conductive surface. Then, at least some of the tin oxide is reduced according to any suitable method to form nanoparticles comprising tin, associated with high surface area carbon, thereby making the electrocatalytic electrode.

Further embodiments relate to methods for reducing carbon dioxide to formate. In some cases, those methods involve providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises: nanoparticles comprising tin, associated with high surface area carbon; exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition; applying a catalyzing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react; thereby reducing the carbon dioxide to formate. Any suitable electrocatalytic cell can be used. In certain instances, such a cell provides a three-electrode configuration with the electrocatalytic electrode functioning as the working electrode, a counter electrode comprising any suitable material such as, for example, platinum, and a reference electrode such as SCE. In other instances, a two-electrode cell is possible. The electrocatalytic electrode is exposed to any suitable fluid composition sufficient to allow carbon dioxide to react at the electrode. In some cases, the fluid composition is aqueous liquid. In certain cases, the fluid composition comprises one or more carbonate salts, one or more bicarbonate salts, one or more phosphate salts, one or more biphosphate salts, or a combination thereof. Still other cases provide a fluid composition comprising potassium bicarbonate, sodium bicarbonate, or a combination thereof. The fluid composition can contain any suitable concentration of carbon dioxide. In some cases, the fluid composition is saturated with carbon dioxide. Any suitable catalyzing potential can be applied to allow the carbon dioxide to react. For example, the catalyzing potential can be more negative than about −1 V versus SCE, more negative than about −1.2 V versus SCE, no more negative than about −1.8 V versus SCE, or no more negative than about −2.0 V versus SCE. In select instances, the catalyzing potential is about −1.8 V versus SCE.

Without wishing to be bound by theory, the practice of various embodiments of the present invention will be illustrated by a description of the experiments performed in our electrochemical study and an analysis of our observations.

Synthesis and Characterization of Nano Tin Oxide.

Tin oxide nanoparticles were synthesized by a modification of a facile hydrothermal method. FIGS. 1a and 1b show TEM images of synthesized tin oxide nanoparticles loaded onto carbon black (ca. 30 nm in diameter), at different resolutions illustrating tin oxide nanoparticles (small black dots) of ˜5 nm in average diameter, uniformly deposited on the surface of the carbon black. In the high resolution TEM images in FIGS. 14a and 14b , obvious lattice fringes of tin oxide are observed and average lattice spacing of ˜0.335 nm measured consistent with the (110) plane of rutile Sn(IV) oxide nanoparticles. The available evidence is all consistent with formation of Sn(IV) oxide nanoparticles (SnO₂). Without wishing to be bound by theory, a formation mechanism is proposed in FIG. 15. It features prior coordination to the SnCl₂ precursor by ethylene glycol (EG) and water with displacement of Cl⁻. At high temperatures aqueated Sn(II) forms Sn(OH)₂ which is converted into SnO and further oxidized by oxygen in the air to give nano-SnO₂.

The high resolution Sn3d XPS spectrum in FIG. 2 exhibits binding energies at 496.5 eV and 488.0 eV that can be assigned to Sn3d_(3/2) and Sn3d_(5/2) ionizations, respectively. These energies are consistent with Sn(IV) bound to oxygen in SnO₂. FIG. 16a shows the survey XPS spectrum for nano-SnO₂/carbon black. The peaks can be assigned to the elements of Sn, In, C, and O, where the presence of element In is due to the background. In the high resolution XPS valence band spectrum in FIG. 16d , three binding energies appear above 5 eV consistent with SnO₂ with two binding energies anticipated for Sn metal and four for SnO. In the Raman spectrum in FIG. 3, characteristic bands appear at 1350 cm⁻¹ and 1580 cm⁻¹, which arise from the D and G bands of carbon black, respectively. The scattering feature at ˜630 cm⁻¹ is a characteristic of rutile SnO₂ nanocrystals.

The three intense diffraction peaks in the XRD pattern shown in FIG. 17 at 2θ=26.2, 33.7, and 51.2 degrees can be indexed as the (110), (101), and (211) planes of the polycrystalline rutile SnO₂ structure, respectively.

FIG. 18 presents an enlarged scattering Raman spectrum of nano-SnO₂ on carbon black. The average particle size of the SnO₂ nanocrystals can be calculated by using the Scherrer equation, eq 1:

$\begin{matrix} {L = \frac{0.9\lambda_{K\; \alpha \; 1}}{B_{2\theta}\cos \; \theta_{\max}}} & (1) \end{matrix}$

In this equation, L is the mean particle size, λ_(Kα1) is the X-ray wavelength (λ=1.5418 Å), θmax is the angle for the (110) plane, and B_(2θ) is the band width at half height for the Pt(110) plane. The XRD patterns are noisy in appearance due to the presence of carbon support. To get a more accurate value of nano-SnO₂ particle size, these patterns had been smoothed using Gaussian Fitting before the average particle sizes of nano SnO₂ were calculated based on the Scherrer equation. Based on this analysis, the SnO₂ volume averaged diameter from the XRD data was ˜4.8 nm which is slightly less than the calculated volume area average diameters from TEM. The discrepancy probably arises from the fact that SnO₂ nanoparticles that are smaller than 1 nm are poorly shown in TEM images. If included, they would decrease the averaged diameters from TEM images.

Electrocatalytic Reduction of CO₂.

Electrocatalytic carbon dioxide reduction by nano-SnO₂ was evaluated in 0.1 M NaHCO₃ aqueous solutions. In the linear sweep voltammetric (LSV) scans in FIG. 19, a reduction peak appears at −1.0 V in initial scans. It arises from reduction of SnO₂ nanoparticles and disappears after 50 cycles from −1 to −1.8 V indicative of reduction of tin oxide to metallic tin on carbon black. It should be noted that all LSV scans and controlled potential electrolysis experiments in this application were conducted on reduced nano-SnO₂ catalysts. A small amount of tin oxide might still persist within the core of the reduced Sn catalyst, in some embodiments. However, due to lack of access to electrolyte, the residual tin oxide presumably does not contribute to CO₂ reduction in those embodiments.

FIG. 7 illustrates single reductive LSV scans at a reduced nano-SnO₂/carbon black coated glassy carbon electrode (0.07 cm²) under 1 atm N₂ and CO₂ saturated 0.1 M NaHCO₃. That electrode is compared to a bare glassy carbon electrode (FIG. 4), carbon black (FIG. 5), and electrodeposited ˜200 nm tin nanoparticles on glassy carbon (FIG. 6). The LSV scans in FIGS. 4 and 5 show that glassy carbon electrodes and carbon black have negligible catalytic currents for CO₂ reduction. The scans in FIG. 6 provide obvious evidence for catalytic reduction of CO₂ at electrodeposited tin nanoparticles with a current density of 3.6 mA cm⁻² at −1.8 V in solutions saturated in CO₂. The current level is nearly twice that of the background but still comparable to that of commercial tin foil under CO₂ (FIG. 20). FIG. 20 presents single reductive linear sweep voltammetric (LSV) scans with scan rate of 50 mV s⁻¹ at tin foil in N₂ (thin line) and CO₂ (thick line) saturated 0.1 M NaHCO₃ solution. The current density at −1.8 V was about 3.2 mA cm⁻² and the corresponding specific current density was about 0.05 A g⁻¹. Prior to electrochemical measurements, tin foil was pretreated in 1 M HNO₃ for 30 s to remove surface contaminants.

By comparison, the current density in FIG. 7 for the reduced nano-SnO₂/carbon black electrode unexpectedly reached 6.2 mA cm⁻² at −1.8 V. When these SnO₂ nanoparticles were loaded onto graphene, their performance for CO₂ reduction was further enhanced with current densities of 13.1 mA cm⁻² reached under the same conditions (see FIG. 13). The specific current densities (with catalytic currents normalized to the mass of Sn catalysts) were calculated to be 266 A g⁻¹ for nano-SnO₂/graphene, 126 A g⁻¹ for nano-SnO₂/carbon black, and 2 A g⁻¹ for electrodeposited tin nanoparticles. Thus, in some embodiments, the nanoparticles comprising tin associated with high surface area carbon exhibit a specific current density greater than about 10 A g⁻¹, or greater than about 100 A g⁻¹, normalized to the mass of Sn catalyst.

Controlled potential electrolyses were performed to investigate the effect of applied potentials on Faradaic efficiencies for formate production at reduced nano-SnO₂/carbon black electrodes. FIG. 8 illustrates the applied electrolysis potential dependence of current densities and Faradaic efficiencies for formate production. As expected, current densities increase with applied overpotentials. Formate anion as an electrolysis product was detected by ¹H NMR.

The results in FIG. 8 show that current efficiencies for formate reach a maximum (86.2%) at moderately negative potentials (−1.8 V) but decrease at more negative potentials. Given E^(o)′=−0.67 V vs. SCE for the CO₂/HCOO⁻ couple, the overpotential for CO₂ reduction under these conditions is 1.13 V. Analysis of gaseous products (Varian 450-GC) after electrolysis revealed that hydrogen was produced at all applied potentials along with small amounts of carbon monoxide (2-6%). At applied potentials more positive than −1.8 V, slow reduction to formate occurs. At very negative potentials, currents increase and the Faradaic efficiency for formate production decreases due to an increase in hydrogen evolution which competes with CO₂ reduction to formate under these conditions.

As shown in FIG. 21, controlled potential electrolysis a nano-SnO₂/carbon black at −1.8 V for 18 hours in 0.1 M NaHCO₃ solution with CO₂ flow resulted in a steady state catalytic current density of ca. 5.4 mA cm⁻². It is to be understood that “SnO₂” when labeling the nanoparticles throughout this application refers to Sn if the nanoparticles have been exposed to applied potentials more negative than about −1 V vs. SCE. See FIG. 19. TEM images in FIG. 22 and LSV curves in FIG. 23 at the reduced nano-SnO₂/carbon black electrode after electrolysis suggest that the morphology and catalytic properties of the SnO₂ nanoparticles are essentially unchanged compared with the initial nano SnO₂/carbon black sample. FIG. 23 plots single reductive LSV scans at nano SnO₂/carbon black sample before and after bulk electrolysis, which showed the catalytic property of nano-SnO₂/carbon black was essentially unchanged after controlled potential electrolysis. It is worth noting that even at the very low overpotential of ˜0.34 V (E_(app)=−1 V vs. SCE, −0.76 V vs. SHE or −0.36 V vs. RHE), formate was still produced although with the relatively low current efficiency of ˜8%.

Electrolysis at SnO₂/graphene was also conducted at −1.8 V for 18 hours in 0.1 M NaHCO₃, FIG. 35. Current densities increased to ˜10.2 mA cm⁻² compared to carbon black as the substrate. More notably, the Faradaic efficiency for formate production at the reduced nano-SnO₂/graphene increased to 93.6%.

CO₂ reduction efficiencies on Sn catalysts depend on particle size. As shown in FIG. 9, Faradaic efficiencies for formate production at −1.8 V at Sn electrodes with varying morphologies show that the maximum current efficiency occurs at 5 nm tin oxide nanoparticles. The same trend was observed at −1.6 V and −2 V (see Table 1 for details). Morphologies for these samples are shown in the SEM and TEM images appearing in FIGS. 24-29.

TABLE 1 Faradaic efficiencies for formate over Sn catalysts from controlled potential electrolyses. Faradaic Electrodeposited efficiencies 3 nm 5 nm 10 nm 200 nm Sn for formate SnO₂ SnO₂ SnO₂ nanoparticles Sn foil −1.6 V 40 51 41 29 20 −1.8 V 64 86 60 35 28 −2.0 V 33 39 32 26 18

To explore a possible role for adsorption of the reduced intermediate CO₂.⁻ during CO₂ catalytic reduction cycles, and how it varies with particle size, we examined adsorption of OH⁻ as a surrogate for CO₂.⁻ as a function of particle size. In these experiments, the current response was monitored following single oxidative LSV scans beginning at −1.6 V vs. SCE through the surface wave for Sn(0) oxidation to Sn(II). Based on the data in FIG. 10, the potential for surface adsorption of OH⁻ is lowest (most negative) for the 3 nm tin oxide nanoparticles and increases positively with increasing particle size. The size dependence suggests an enhanced binding energy for OH⁻ on Sn(II)-doped surfaces which is maximized for the 3 nm tin oxide particles.

In comparing the data in FIGS. 9 and 10, enhanced surface binding does not correlate with enhanced electrocatalytic activity toward CO₂ reduction with the maximum efficiency reached for 5 nm particles. For CO₂ reduction, without wishing to be bound by theory, maximized efficiencies for 5 nm tin oxide nanocrystals may arise from an optimized affinity toward surface binding of a key intermediate or intermediates during CO₂ reduction. In particular, adsorption of electrogenerated CO₂.⁻ may play a key role in the initial step with E^(o)=−2.21 V vs. SCE in dry dimethylformamide (DMF) for the CO₂/CO₂.⁻ couple. An additional factor may be that competitive hydrogen evolution is suppressed at 5 nm tin oxide nanocrystals.

A possible CO₂ reduction mechanism on Sn is illustrated in eqs 1-5. In the reaction sequence, eq 3 is presumably the rate-determining step (RDS). This assignment is supported by the results in FIGS. 30 and 31 showing that catalytic current densities for CO₂ reduction increase essentially linearly with the concentrations of HCO₃ ⁻. FIG. 30 plots single reductive LSV scans at 5 nm reduced tin oxide catalyst in different concentrated CO₂ saturated NaHCO₃ electrolyte, where total concentration of Na⁺ was kept at 0.2 M by adding NaClO₄. Under these conditions, NaHCO₃/CO₂ is the buffer. The pH values slightly increased from 6.2 to 6.9 when the NaHCO₃ concentrations were increased from 10 mM to 200 mM. Without any NaHCO₃, the pH is ca. 5.5 under 1 atm of CO₂. FIG. 31 plots NaHCO₃ concentration dependence of current density at −1.8 V, which showed catalytic current densities for CO₂ reduction nearly linearly increase with concentrations of HCO₃ ⁻ in the electrolyte. We propose that HCO₃ ⁻ is the likely source as the dominant proton donor based on its pKa value (10.33) compared to water (15.7). Carbonic acid in the CO₂ saturated solutions is present but in small amount.

Additional evidence is provided by the Tafel slope (FIG. 32) of ˜70 mV dec⁻¹, which is close to 59 mV dec⁻¹. FIG. 32 depicts a Tafel plot for production of formate at 5 nm SnO₂ sample based on the data in FIG. 8, for production of formate at 5 nm SnO₂. The Tafel slope suggests a mechanism involving a chemical rate-determining step rather than an initial rate-determining transfer of one electron to CO₂. These kinetics are consistent with electron transfer and adsorption of CO₂.⁻ followed by rate limiting proton transfer from HCO₃ ⁻. Proton transfer may trigger a second electron transfer from the electrode to give the adsorbed formate product. Alternately, concerted electron-proton reduction of adsorbed CO₂.⁻ with electron transfer from the electrode occurring in concert with proton transfer from HCO₃ ⁻, eq 3, may occur with the advantage of giving adsorbed formate directly.

In these mechanisms, surface adsorption plays a role in electrocatalytic CO₂ reduction. Surface stabilization by adsorption would increase the barrier to further reduction and protonation of CO₂.⁻ with weak adsorption decreasing the activating effect of surface binding. Experimentally, the interplay between the two leads to maximized efficiencies for 5 nm particles, in some embodiments.

CO₂(solution)→CO₂(ads)  (1)

CO₂(ads)+e ⁻→CO₂.⁻(ads)  (2)

CO₂.⁻(ads)+HCO₃ ⁻ +e ⁻→HCO₂ ⁻(ads)+CO₃ ²⁻  (3)

HCO₂ ⁻(ads)→HCO₂ ⁻(solution)  (4)

CO₃ ²⁻+CO₂+H₂O→2HCO₃ ⁻  (5)

The electronic structure of the nano tin catalyst may also play an role with catalysis maximized on graphene as the support. Graphene is a two dimensional, one atom thick sp²-bonded delocalized carbon substrate. Strong electronic interactions between graphene and added metal nanoparticles can modify the electronic structure of the latter and influence surface molecular adsorption and reactivity, in some embodiments of the present invention.

FIGS. 33a and 33b provide TEM images of nano-SnO₂/graphene sample, which showed SnO₂ nanoparticles with ˜5 nm diameter that are uniformly loaded on the surface of graphene. In contrast to the monodisperse SnO₂ nanoparticles in SnO₂/carbon black, there is an increase in the interconnections between SnO₂ nanoparticles to form SnO₂ nanoparticle clusters in the SnO₂/graphene samples. This may be due to the higher content of oxygen-containing defects in graphene used in this study (˜8 at. % determined by XPS) than that in carbon black (˜1 at. %).

FIG. 34 provides (a) XRD and (b) survey XPS spectra of an unreduced nano-SnO₂/graphene sample. Three intense diffraction peaks in the XRD pattern shown in FIG. 34(a) at 2θ=25.6, 32.8, and 50.9 degrees can be indexed as the (110), (101), and (211) planes of the polycrystalline rutile SnO₂ structure, respectively. The XRD patterns are noisy in appearance due to the presence of carbon support. To get a more accurate value of nano-SnO₂ particle size, these patterns had been smoothed using Gaussian Fitting before the average particle sizes of nano SnO₂ were calculated based on the Scherrer equation. The average particle size of the SnO₂ nanocrystals on graphene can be calculated by using the Scherrer equation to be ˜4.9 nm.

FIG. 35 depicts controlled potential electrolysis at −1.8 vs. SCE at the reduced nano-SnO₂/graphene in 0.1 M NaHCO₃ solution with CO₂ flow. The reduced nano-SnO₂/graphene reaches higher Faradaic efficiencies for formate production (93.6%) at nearly twice the current density (10.2 mA cm⁻²) as at SnO₂/carbon black.

The TEM images in FIG. 11a and FIG. 33 show nano-SnO₂ particles of ˜5 nm diameter that are loaded on the surface of graphene. In the high resolution TEM image in FIG. 11b , clear lattice fringes are observed with average lattice spacing of ˜0.335 nm for the (110) plane of rutile nano-SnO₂ materials. SnO₂ nanoparticles in SnO₂/carbon black and SnO₂/graphene were prepared by the same method with the same particle size, crystallinity (Figure S34), and catalyst loading (˜30 wt. % as determined by Energy Dispersive Spectroscopy). However, as shown in FIG. 35 at −1.8 V, higher Faradaic efficiencies for formate production (93.6%) were obtained at reduced nano-SnO₂/graphene, nearly twice the current density (10.2 mA cm⁻²) than for SnO₂/carbon black in FIG. 21.

The difference in catalytic activity may be a consequence of differences in the influence of electronic perturbations on Sn particle electronic structure on the two supports. From the high resolution Sn3d XPS spectrum in FIG. 12, Sn 3d XPS binding energies appear at 496.2 eV and 487.7 eV, respectively, shifted 0.3 eV to lower energy compared to SnO₂/carbon black in FIG. 2. The negative shift in binding energies may arise from the stronger electron donating ability of graphene compared to carbon black. Without wishing to be bound by theory, this electronic interaction may lead to enhanced electronic donation promoting adsorption of CO₂ and CO₂.⁻ and facilitating CO₂ reduction at the Sn surface.

Further embodiments of the present invention provide tin oxide nanoparticles of varying size. By controlling the size of tin oxide nanoparticles on carbon supports, overpotentials as low as ˜340 mV can be achieved for CO₂ reduction to HCOO⁻ with significant enhancements in both current density and efficiency. In aqueous NaHCO₃ solutions saturated in CO₂, maximum Faradaic efficiencies for formate production of >93% have been reached with current densities of over 10 mA cm⁻² on high surface area graphene supports. The reduced nano tin oxide (i.e., metallic tin) catalysts are highly stable during controlled potential electrolysis and it is contemplated to achieve current density enhancements of 1-2 orders of magnitude by using flow cell or gas diffusion electrodes (GDE).

The reactivity toward CO₂ reduction achieved here is notable. It may arise from a compromise between the strength of the interaction between CO₂.⁻ with the nano tin surface and its subsequent kinetic activation toward protonation and further reduction. With the graphene support there may also be a role for electronic interactions with the underlying graphene substrate. In any case, our results demonstrate important roles for catalyst morphology and electronic structure in the electrocatalytic reduction of CO₂.

Examples 1 Materials Preparation

Preparation of Nano Tin Oxide Catalysts.

Tin oxide nanoparticles were synthesized by a modification of a facile hydrothermal method. 100 mg of Tin(II) chloride (SnCl₂, 98%, Acros Organics) precursor was dissolved in ethylene glycol (EG, Fisher Scientific) with trace amount of water, mixed with 160 mg of carbon support (carbon black, VULCAN® XC72, purchased from Cabot Corporation; or graphene prepared by a thermal expansion method reported in the literature), and then ultrasonicated for ˜30 min. The resulting mixture was heated up to 196° C. and then refluxed with vigorous stirring. The product was collected by ultrafiltration and washed with deionized water, and then dried for 3 h at 90° C. in vacuum. Particle size of the resultant tin oxide can be tuned by reaction time. When reaction time proceeded in 30 min, about 3 nm tin oxide nanoparticles were formed. Around 5 nm tin oxide nanoparticles were obtained when the reaction time was increased to 3 hours. The largest tin oxide nanoparticles prepared here were about 10 nm after 6 hours.

Electrodeposited Tin Particles on Pre-Polished Glassy Carbon Electrode.

A glassy carbon electrode was first well polished in 0.3 and 0.05 micro Alumina powder to obtain a mirror surface and then sonicated and thoroughly rinsed with Milli-Q water and acetone. The polished glassy carbon electrode as the working electrode was immersed into 10 mM SnCl₂ dissolved in acetonitrile with 0.1 M TBAPF₆ electrolyte, and then was scanned for 5 cycles from −1 to −2 V versus SCE with a Pt wire counter electrode. Based on CV curves described above, about 0.2 C of charge was consumed during the reduction of SnCl₂ to Sn(0) giving the loading of electrodeposited Sn on glassy carbon to be about 123 μg.

Tin Foil.

Commercial tin foil (1.7 g/50×50 mm), 0.25 mm (0.01 in) thick, 99.998% (metals basis) was purchased from Alfa Aesar. Before electrochemical measurements, the as-received tin foil (about 3400 μg) was pretreated in 1 M HNO₃ for 10 second to remove surface oxide and contaminants.

2 Electrochemical Measurements

The electrochemical measurements were carried out in a gas-tight two compartment electrochemical cell system controlled with a CHI601 D station (CH Instruments, Inc., USA) with Pt wire and Saturated Calomel Reference Electrode (SCE) as the counter electrode and reference electrode, respectively. The working electrodes were prepared by loading sample suspension onto the prepolished glassy carbon electrodes. In brief, as prepared nano-SnO₂/carbon black or nano-SnO₂/graphene sample was dispersed in ethanol and ultrasonicated for 15 min to form a uniform catalyst suspension (2 mg mL⁻¹). A total of 7.5 μL of well dispersed catalyst ink was applied onto the pre-polished glassy carbon electrode (3 mm in diameter). This gives the loading of carbon supported nano-SnO₂ sample to be about 15 μg, and the amount of Sn is calculated to be about 3.45 μg based on the nano-SnO₂ content of ˜30% in catalyst. After drying at room temperature, 5 mL of 0.05 wt % Nafion solution was applied onto the surface of the catalyst layer to form a thin protective film. The well-prepared electrodes were dried at room temperature overnight before the electrochemical tests. Linear sweep voltammetric (LSV) scans were recorded in N₂ or CO₂ saturated 0.1 M NaHCO₃, while controlled potential electrolysis was performed in 0.1 M NaHCO₃ electrolyte with CO₂ flow.

3 Physical Characterizations

Raman spectra were collected by Raman measurements (Renishaw) with a 514 nm laser. X-ray data were collected on Bruker Smart APEX II CCD diffractometer equipped with Cu-target X-ray tube and operated at 1600 watts. High-resolution TEM (HRTEM) was obtained on a JEOL 2010F-FasTEM. Scanning electron microscopy (SEM) and energy dispersive X-ray spectrometer (EDS) were obtained on a FEI Helios 600 Nanolab Dual Beam System focused ion beam (FIB) equipped with an Oxford Instruments, INCA PentaFET-x3 X-ray detector with the electron beam set to 20 keV and a beam current of 0.69 nA. In addition, the ion beam was used to mill into the film and determine its permeability to the deposited metal particles. EDS measurements were made of film surfaces, and with a 1 μm², 100 nm deep square milled into the film, another EDS spectrum was recorded.

NMR analysis was used to quantify the yield of formate during controlled potential electrolysis. NMR spectra were recorded on Bruker NMR spectrometers (AVANCE-400). 1H NMR spectra were referenced to residual solvent signals. At the end of electrolysis periods, gaseous samples (0.8 ml) were drawn from the headspace by a gas-tight syringe (Vici) and injected into the GC (Varian 450-GC, pulsed discharge helium ionization detector, PDHID). Calibration curves for H₂ and CO were determined separately.

X-ray photoelectron spectra (XPS) were obtained at the Chapel Hill Analytical and Nanofabrication Lab (CHANL) at UNC. A Kratos Analytical Axis UltraDLD spectrometer with monochromatized X-ray Al Kα radiation (1486.6 eV) with an analysis area of 1 mm² was used. A survey scan was first performed with a step size of 1 eV, a pass energy of 80 eV, and a dwell time of 200 ms. High resolution scans were then taken for each element present with a step size of 0.1 eV and a pass energy of 20 eV. The binding energy for all peaks was referenced to the C 1s peak at 284.6 eV XRD.

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As previously stated, detailed embodiments of the present invention are disclosed herein; however, it is to be understood that the disclosed embodiments are merely exemplary of the invention that may be embodied in various forms. It will be appreciated that many modifications and other variations stand within the intended scope of this invention as claimed below. Furthermore, the foregoing description of various embodiments does not necessarily imply exclusion. For example, “some” embodiments may include all or part of “other” and “further” embodiments within the scope of this invention. In addition, “a” does not mean “one and only one;” “a” can mean “one and more than one.” 

We claim:
 1. A method for reducing carbon dioxide to formate, comprising: providing an electrocatalytic electrode in a suitable electrocatalytic cell, wherein the electrocatalytic electrode comprises: nanoparticles comprising tin, associated with high surface area carbon; exposing the electrocatalytic electrode to a concentration of carbon dioxide in a fluid composition; applying a catalyzing potential to the electrocatalytic electrode and allowing at least some of the carbon dioxide to react; thereby reducing the carbon dioxide to formate.
 2. The method of claim 1, wherein the fluid composition comprises potassium bicarbonate, sodium bicarbonate, or a combination thereof.
 3. The method of claim 1, wherein the catalyzing potential is about −1.8 V versus SCE.
 4. The method of claim 1, wherein the nanoparticles comprising tin, associated with high surface area carbon exhibit a specific current density greater than about 10 A g⁻¹.
 5. The method of claim 1, wherein the nanoparticles comprising tin, associated with high surface are carbon exhibit a specific current density greater than about 100 A g⁻¹.
 6. An electrocatalytic electrode, comprising: an electrically-conductive surface in electrical communication with nanoparticles comprising tin, associated with high surface area carbon.
 7. The electrocatalytic electrode of claim 6, wherein the high surface area carbon is chosen from carbon nanotubes, carbon black, mesoporous carbon, graphite, graphene, and combinations of two or more thereof.
 8. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size of about 200 nm or less.
 9. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size of about 10 nm or less.
 10. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size ranging from about 3 nm to about 10 nm.
 11. The electrocatalytic electrode of claim 6, wherein the nanoparticles have an average size of about 5 nm.
 12. The electrocatalytic electrode of claim 6, wherein the electrically-conductive surface comprises glassy carbon, carbon paper, carbon cloth, or a combination thereof.
 13. The electrocatalytic electrode of claim 6, wherein the electrically-conductive surface forms part of a gas diffusion electrode.
 14. A method for making an electrocatalytic electrode, comprising: depositing, on an electrically-conductive surface, nanoparticles of tin oxide associated with high surface area carbon; reducing at least some of the tin oxide to form nanoparticles comprising tin, associated with high surface area carbon, thereby making the electrocatalytic electrode.
 15. The method of claim 14, further comprising: reacting tin(II) chloride in the presence of high surface area carbon and water for a time, thereby forming the nanoparticles of tin oxide associated with high surface area carbon.
 16. The method of claim 15, wherein the time is at least 30 minutes.
 17. The method of claim 15, wherein the time is at least 3 hours.
 18. The method of claim 15, wherein the time is no more than about 6 hours.
 19. The method of claim 14, wherein the reducing comprises applying a reducing potential to the tin oxide no more positive than about −1 V versus SCE.
 20. The method of claim 14, wherein the nanoparticles of tin oxide comprise rutile tin oxide. 